A foamed molding obtained by heating and foaming expandable resin beads such as expandable polystyrene beads or expandable polyolefin beads filled in a mold has been traditionally used widely for containers of fresh foods, such as, fish cases, transport boxes and buffer material. Considering the applications, heat insulation and resistance to impact properties are important, and most materials are substantially non-permeable, a porous structure suited to sound absorbing material is hardly known.
It has been studied to develop a sound absorbing material by forming a proper porous structure in a foamed resin molding, but to realize a preferred porosity for sound absorbing material, the heating temperature must be lowered, but the bond strength of foamed cells lowers, and practical structural strength as a molding is not obtained, and practical material has not been developed. By solving the problems, a sound absorbing material achieving both porous structure desired and the structural strength suitable for sound absorbing material is disclosed in patent document 1.
The sound absorbing material in patent document 1 is made of expandable resin beads, and is manufactured by adhering a heat-sealable resin sealable at lower temperature than the softening and foaming temperature of the particles to the surface, adjusting they foaming amount when molding so as to leave a porous structure, and adhering and bonding the foamed cells each other by this adhesive resin.
However, since the adhesive resin is used, this sound absorbing material had the following practical problems.
1. The material cost and processing cost are increased because of the additional adhesive resin.
2. The fluidity of expandable resin beads is lowered by the adhesive resin, and the filling device may be clogged, the filling degree in the mold is not uniform, and the operability is inferior.
3. The heat resistance and long-term durability of the sound absorbing material are substantially lowered due to low temperature softening characteristic of the adhesive resin.    Patent document 1: Japanese Patent Publication No. 3268094